Wafer polishing machines generally include a turntable that may be driven in rotation about a vertical axis passing through the center of the turntable. A replaceable polishing pad may be mounted on an upper surface of the turntable. A wafer to be polished is held by a wafer carrier attached to a rotatable polishing head. Some wafer polishing machines include more than one polishing head and some polishing machines for polishing wafers in batches have polishing heads that are adapted for attachment of more than one wafer carrier. Wafer polishing may be performed by lowering a polishing head until a surface to be polished on each wafer contacts an upper surface of the rotating polishing pad. Polishing slurry, which may include chemical polishing agents and abrasive particles, may be applied to the polishing pad.
To achieve a high quality of wafer polishing, where high quality polishing generally refers to forming a uniformly flat, smooth surface on a wafer, a wafer to be polished may be pressed into the polishing pad with a large normal force. In some previously known wafer polishing machines, a lower part of a polishing head may be connected to an upper part with a spherical joint as in, for example, U.S. Pat. No. 4,194,324 to Bonora et al. The spherical joint, sometimes referred to as a ball-and-socket joint, includes a shaft ending in a spherical socket which fits securely over a convex spherical surface of a lower part of the polishing head. The ball-and-socket joint enables a wafer carrier attached to the polishing head to tilt around relative to the common center of the spherical surface. A wafer attached to the wafer carrier is therefore able to maintain contact with the polishing pad across the entire lower surface of the wafer.
The coefficient of friction between the polishing pad and the wafer being polished may be large. The coefficient of friction and the normal force pressing the wafer into the polishing pad may result in a large frictional force directed horizontally, that is, approximately parallel to the working surface of the polishing pad. For previously known wafer polishing machines having a spherical joint in a polishing head, there may be a vertical separation distance between the rotational center of the spherical joint and the working surface of the polishing pad. The large frictional force and the vertical separation distance between the center of rotation of the spherical joint and the surface of the polishing pad may result in a large torque force in a vertical plane being applied to a wafer carrier. The torque force may result in undesirable deviations from flatness of a wafer's polished surface. For example, the torque force may increase pressure between a wafer and polishing pad along the leading edge of the wafer and decrease pressure along the trailing edge of the wafer as the wafer and wafer carrier move across the polishing pad by rotational motions of the polishing head and turntable. The difference in pressure between a wafer's leading and trailing edges may result in a polished surface which is not sufficiently flat, corresponding to a reduction in polishing quality. The pressure difference may also reduce the service lifetime of the polishing pad.
A wafer polishing machine having more than one wafer on each wafer carrier may be able to perform batch processing, that is, simultaneous polishing of a batch comprising more than one wafer. For previously known wafer polishing machines, material may be removed more quickly from a thicker wafer than from other, thinner wafers attached to the same wafer carrier. Differences in rates of material removal from wafer to wafer may result in undesirable differences in polishing quality between wafers. It is therefore known to sort wafers into batches with each batch having a specified range of wafer thickness. The range of wafer thickness for each batch may be related to variations in wafer flatness within each batch. Even with wafer sorting, some undesirable variation in polishing quality, such as variations in wafer flatness, may still occur from wafer to wafer. Wafer thickness sorting may therefore lead to a compromise in the quality of polished wafers. Furthermore, labor costs for wafer sorting and costs for purchasing, installing, operating, and maintaining wafer thickness measurement equipment add to the cost of polished wafers.
For some previously known wafer polishing machines with more than one wafer per carrier, each wafer cannot freely rotate around its own axis. Parts of a wafer that are closer to the rotational center of a polishing head may therefore be polished at a different rate than parts of the wafer that are farther from the rotational center. Differences in contact pressure between the wafers on different wafer carriers and the polishing pad and variations in slurry distribution from one wafer to another may also cause variations in the quality of polished wafers.
Efforts have been made to reduce the vertical separation distance between the rotational center of the spherical joint and the surface of the polishing pad. See for example U.S. Pat. No. 5,377,451 to Leoni et al. and U.S. Pat. No. 7,137,874 to Bovio et al. A previously known method for reducing the vertical separation distance is to increase the radii of the pivoting spherical surfaces in the spherical joint in a polishing head. Another previously known method is to replace sliding bearings with rolling bearings. Yet another previously known method is to encapsulate the outside part of a spherical bearing with a spherical surface formed into part of a wafer carrier. However, friction from the encapsulating spherical surface may increase torque on the wafer carrier, causing undesirable polishing variations across wafer surfaces.
A flexible boot may be used in some wafer polishing machines for rotationally driving a wafer carrier. However, the stiffness of the boot may also increase torque in a vertical plane on the carrier. The increase in torque from the flexible boot reduces the effectiveness of reducing the vertical separation distance between the rotational center of the spherical joint and the surface of the polishing pad. The increase in torque further causes a geometric point about which the carrier may tilt and rotate, the geometric point being referred to herein as a gimbal point, to be displaced from the geometric center of the spherical surfaces in the spherical joint. The attribute of flexibility in a boot, with high flexibility preferred for uniform polishing of all the wafers in a polishing batch, and the attribute of rigidity in the boot, with high rigidity preferred for predictable, controllable rotation of a wafer carrier are in opposition to each other for high quality polishing and may lead to conflicting requirements for polishing parameters.
For previously known wafer polishing machines, it may be difficult to predict how the gimbal point will be displaced from the rotational center of the spherical joint. It may therefore be difficult to set up a wafer polishing machine to achieve desired polishing results. Furthermore, an optimum value for the vertical separation distance between the gimbal point and the working surface of the polishing pad may depend on parameters such as radii of the pivoting spherical surfaces in the spherical joint, pressure applied to the wafers, wafer diameter, type of polishing slurry, polishing slurry flow rate, polishing pad material, rates of rotation of the polishing heads and turntable, and other factors. For a polishing machine with a fixed relationship between the separation of the gimbal point and polishing pad, optimal polishing conditions may be achieved for one selected set of operational parameters, but polishing with different parameters may result in suboptimal polishing.